Technical Report NTB 21-02

Abstract Nagra's mission is to develop and implement safe and sustainable long-term solutions for all radio­active waste arising in Switzerland from the use of nuclear energy and from medicine, industry and research in a timely manner and at reasonable cost. The applications for the general licences required for the repositories will be prepared and submitted within the framework of the Sectoral Plan for Deep Geological Repositories (Sachplan geologische Tiefenlager). According to Art. 31 par. 1 of the Swiss Nuclear Energy Act (NEA 2003), the operators of nuclear installations are obliged to safely manage all radioactive waste arising from their installations. This includes the necessary preliminary activities such as research and geological investigations. Article 36 of the Swiss Nuclear Energy Ordinance (NEO 2004) specifies the research activities. Art. 36 par. 2 NEO states that the licence holders must monitor technological developments, including those relating to organisation and personnel, and must examine the extent to which conclusions may be drawn concerning the safety and security of the licence holder's installation, in Nagra's case the geological repositories. In a decision of August 2013, the Federal Council concluded that Nagra must submit a Research, Development and Demonstration Plan (RD&D Plan) together with the Waste Management Programme every five years. The present RD&D Plan is in line with the Federal Council's decision and with the regulatory framework in Switzerland. The RD&D Plan documents the objectives, scope, nature and timing of future RD&D activities and describes how existing open questions have been addressed. It outlines both a strategic plan for Nagra's RD&D activities contributing to the implementation of a combined repository or two separate repositories for high-level waste (HLW) and low- and intermediate-level waste (L/ILW) as well as a technical work programme consistent with the strategic objectives. Part 1 – Chapters 1 – 5 The first part of this report focuses on providing the programme context, i.e. the roadmap for the stepwise implementation of the combined or separate HLW and L/ILW repositories and the planning assumptions consistent with these requirements (Chapters 1 – 4). The Nagra Disposal Programme Roadmap (Nagra Roadmap) supports programme planning, coordination and visuali­sation of the expected future programme, based on the current boundary conditions. It is a tool for specifying, planning and communicating the entire, integrated programme. The Roadmap is arranged over three integrated tiers, each with a different focus:

• Tier 1: Programme, captures a high-level description of the Nagra disposal programme all the way up to the return of the site(s) to a greenfield state around 2126. Tier 1 covers the iterative repository design and safety assessment process and the legally binding licensing process. It sets out the overarching framework against which programme activities are mapped at Tier 2 and Tier 3.
• Tier 2: Optimisation, captures the activities at the core of the programme, up to the return to a greenfield state. Specifically, Tier 2 covers the development and optimisation of the repository design from the perspective of safety during construction, operation and post-closure and from the perspective of implementation, to ensure technical feasibility and practicability.
• Tier 3: RD&D, captures the underpinning RD&D activities necessary to optimise and realise a Swiss repository up until first L/ILW emplacement around 2050 and HLW emplacement around 2060. Tier 3 covers RD&D activities required to support specific optimisation work at Tier 2, communicating the necessity and sufficiency of the Nagra RD&D programme.

The specification, planning and implementation of RD&D is a needs-driven process, where the general drivers are the optimisation processes which must be carried out from the perspective of safety (recurring safety cases at each milestone) and implementation (confidence underpinning the feasibility of the design at each milestone). The ongoing development of the repository concept, design and components currently represents an important component of the RD&D activities. Tier 3 of the Roadmap covers all RD&D activities which will be managed by Nagra, but the current focus is on ten specific programme aspects with a significant RD&D component. For these aspects, the Roadmap illustrates how the Nagra RD&D Plan meets the evolving needs of the programme, as the licensing process necessitates an increasing level of refinement in the various elements of the disposal programme. The current status of work is also described in Chapter 5 of Part 1 of this report and includes the progress of the Sectoral Plan process and the preparations for the submission of the general licence applications, currently foreseen for 2024. This also includes the structure of the safety case that will be submitted with the licence applications, and the changes to the Nagra repository project since 2016. The current international status of HLW (and L/ILW) disposal is documented and defines the starting point for the RD&D activities over the remainder of the Nagra disposal programme. Finally, the key principles regarding RD&D resources and competence management are highlighted. Part 2 – Chapters 6 – 8 In the second part of this report, the focus is on assessing and outlining the work required to further develop Nagra's science and engineering base and to ensure that the technology, data and expertise necessary for optimising the concepts and achieving future milestones in repository planning and development, including eventual implementation, are acquired in a timely and cost-effective manner. The scope of the report is thus wider than describing activities that are defined sensu stricto as RD&D. As part of this optimisation process (safety, feasibility, implementation and thus also cost), this also includes the development of alternatives at all levels to ensure flexibility in terms of including future developments in the concepts. Given the very broad nature of the activities involved and the need to keep the document sufficiently succinct, the initial focus is on the planning of the RD&D programme for the remainder of the implementation period up to repository closure and the return to a greenfield state. This is described in Chapter 6 and includes the main objectives of the planned activities in the facilities for underground geological investigations as well as their approximate timing. Chapter 7 describes how Nagra plans to address programme aspects with a significant RD&D component (Tier 3 of the Nagra Roadmap). Much of the RD&D is necessary to contribute towards and underpin future decision-making. While feasibility will be demonstrated at a conceptual level for the general licence applications, there are many areas of the programme where maintaining flexibility is highly beneficial. Such flexibility is particularly valuable in areas where considerable progress is expected in the intermediate term, e.g. specific technological developments and advances in materials science. From a safety or other perspective it could be beneficial to optimise specific solutions initially chosen in the near term once the time for implementation arrives. The individual Tier 3 roadmaps cover RD&D contributions to decision-making and providing confidence in the selection of specific technologies and approaches within the appropriate timeframes. They communicate the need to maintain options and alternatives until the programme approaches a specific milestone, with optimisation as the key driver in each case. These programme aspects with a significant RD&D component that are developed as separate Tier 3 roadmaps are: (1) spent fuel, (2) monitoring, (3) thermal output, (4) gas, (5) geological evolution, (6) hydrochemistry and barriers, (7) L/ILW container, (8) HLW canister, (9) backfilling, sealing and closure, and (10) emplacement and retrievability. Chapter 8 first provides a detailed overview of the progress since the previous RD&D Plan (Nagra 2016b), highlighting and summarising the achievements realised in the various domains described below. It continues with a description of the technical work programme over the next five to ten years. Geological information – The main drivers for the work are (1) the site selection process, (2) the safety case to be developed for the selected site(s), including provision of the basis for assessing repository-induced effects, and (3) the engineering of the subsurface infrastructure.

• The bulk of the RD&D activities foreseen supports the geosphere characterisation in the siting regions and the development of the site-specific models. These are strongly supported by the results of the 3D seismic campaigns and the deep borehole campaign in the siting regions.
  • Characterisation methods for the sedimentary facies have made significant progress, while the tectonic picture for the three siting regions has become clear. Based on the seismic interpretation, the advanced characterisation of sedimentological and tectonic features will continue to refine this.
  • Major progress was achieved through benchmarking of geomechanical properties, while novel techniques for characterising the stress field in the siting regions are being applied and interpreted.
  • The hydrogeological and hydrogeochemical characterisation techniques, already fairly mature, have been further refined, providing increased insight into the expected long-term evolution of the porewater and groundwater systems.
  • The activities providing an understanding of the long-term geological and climate evolution have made a major leap forward and are also supported by an extensive Quaternary sediment characterisation programme. The novel assessment methodology developed is based on the evaluation of fluvial processes, the evolution of local topography and glacial erosion dynamics. It will provide strong support for site selection and for the subsequent description of the long-term evolution of the selected site(s).
• Process understanding of the key safety-relevant phenomena in the containment-providing rock zone was further refined to provide the basis for the description of repository-induced effects and for the safety assessment.
  • In the area of radionuclide transport, where understanding was already highly advanced, concluding syntheses of various aspects (e.g. competitive sorption) are being compiled to underpin the safety assessment.
  • Gas transport characterisation receives considerable attention, also internationally, and has focused most recently on the characterisation of gas-induced fractures. To accommodate remaining uncertainties, a framework was developed to assess gas-related impacts in the siting regions.
  • A more robust geomechanical understanding provides strong support for repository design and is now based on more advanced constitutive models. The impact of material heterogeneity is being further assessed.
  • Understanding of thermal properties and behaviour is key for the thermal dimensioning of the repository and efforts are focused on extensive characterisation, benchmarking of experiments in underground rock laboratories (URLs) and development of probabilistic workflows to support site selection and thermal optimisation.
  • While the state of the art regarding understanding of self-sealing processes in the host rock is mature, novel small-scale visualisation experiments convincingly demonstrate the rapid self-sealing of artificial fractures.

Radioactive waste and materials – Comprehensive knowledge of the radiological and material properties of all waste streams destined for disposal is key to the design and safety analysis of the planned repositories.

• A refined description of the inventory will be prepared for the general licence applications, leading to an update of the respective databases. The state of the art regarding waste conditioning is being followed closely in order to identify waste treatment options for optimisation in future programme stages.
• Further progress was made in reducing uncertainties in the safety analysis related to the waste characteristics:
  • The focus is mainly on the radionuclide release from spent fuel and the release rate of 14C from L/ILW, since reducing these can contribute to optimising safety. International efforts play a major role in targeting specific aspects of the spent fuel release, and long-term experiments focus on 14C release from metallic and organic wastes.
  • While uncertainties with respect to glass dissolution remain and are being further investigated, they do not have consequences for post-closure safety.
  • Uncertainties regarding the gas produced by the waste have been further reduced by increasing confidence that corrosion rates of metals, which are the main source of repository gases, will be very low.
  • To ensure criticality safety, the methodology based on burnup credit was further developed and loading curves were obtained for nominal canisters. To ensure criticality safety in the later stages of repository evolution, the method will be further improved.
• Assessing the state and integrity of spent fuel after dry storage is mainly an issue related to handling and to the design of the waste encapsulation plant. Along with multiple international initiatives, Nagra also contributed significantly to investigations on spent fuel rods and structural materials.
• In the context of the thermal optimisation of the repository, logistics optimisation software has been enhanced to allow for more comprehensive analysis of canister loading options.

Design and technology development – The general feasibility of a repository and its safe operation, backfilling and closure must be demonstrated at the conceptual level for the general licence applications. Multiple aspects of the subsurface infrastructure lie in the domain of engineering and infrastructure development. This applies specifically to the development of generic design concepts and operating schemes and their adaptation first to the specific sites in the three siting regions and ultimately to the selected site(s).

• Based on a deepened analysis of the anticipated behaviour of the rock mass, a shield tunnel boring machine (TBM) with segmental lining support has been chosen as the reference for construction of the emplacement drifts for spent fuel (SF) and HLW. Different tunnel support systems have been successfully tested.
• The buffer emplacement technology for meeting the post-closure safety requirements was already demonstrated at a 1:1 scale in the FE experiment in the Mont Terri Rock Laboratory, and further experience is being gained in large-scale emplacement experiments at the Grimsel Test Site (GTS). Evaluation of alternative, more widely available bentonites that fulfil the requirements continues in the context of the optimisation process.
• The design of the HLW carbon steel disposal canister has progressed. In anticipation of the general licence applications, a larger-scale feasibility demonstration will be pursued. The development of alternative canisters continues, with a major focus on copper-coated canisters that have reached a high level of maturity as a result of international developments.
• The design and prototype production of the L/ILW disposal containers has been completed. Their sizes are now also optimised for interim storage. Options for infilling the containers have been further elaborated.
• The cement-based backfill for the L/ILW emplacement caverns has to be designed to meet the requirements for post-closure safety (radionuclide retention and gas storage capacity). The focus of the work is on selection of the aggregates and the engineering properties of the selected compositions.
• For sealing and closure of the repository, variants that are optimised for post-closure requirements and implementation are being investigated. This is supported by international plug and seal demonstrations and the 1:1 Gas-permeable Seal Test (GAST) at the GTS and its supporting laboratory experiments.

The conceptual design and preparation of the surface facilities is based on standard engineering practice and, in this phase, does not require specific RD&D activities. One exception is the waste encapsulation plant, which still has the potential to be significantly optimised based on the canister loading concepts that will be used in the context of thermal optimisation of the repository design and operation. Evolution of safety-relevant properties and processes in the near-field – The RD&D needs in this domain are driven by the component-specific functions that have to be fulfilled with respect to the post-closure safety assessment. This requires a detailed understanding of repository evolution, both for HLW and L/ILW, which is challenging as the processes are highly coupled and the environments are complex. HLW near-field:

• Understanding of the thermo-hydro-mechanical (THM) evolution has deepened through continuation of a series of large-scale URL experiments and benchmarking of their respective models. A repository-scale THM assessment workflow has been developed to support site selection and repository optimisation.
• Progress in the estimation of corrosion rates under repository-relevant conditions means that reference corrosion rates can now be estimated realistically, resulting in lower gas generation rates in the early stage of the HLW repository evolution.
• The widespread application of bentonite in the designs of advanced international waste management programmes means that understanding of the evolution of the bentonite barrier has reached a high level of maturity, allowing the remaining issues specific to the Swiss concept to be addressed. Higher temperature performance of bentonite is being investigated in the context of the thermal optimisation of the HLW repository.
• The characterisation of the excavation damaged zone (EDZ) based on investigations in the Mont Terri Rock Laboratory has now been summarised and integrated, while upscaling methods of the EDZ have been developed. The role of the EDZ as a preferential flow path for radionuclides was shown to be minimal in a dead-end HLW emplacement drift concept.

L/ILW near-field:

• The thermo-hydro-mechanical workflow mentioned above also extends to the L/ILW near-field to assess the impact of gas generation and the interactions between the HLW and L/ILW sections of a combined repository. The performance confirmation of gas-permeable seals was further developed and will be finalised in anticipation of the general licence applications.
• The cementitious near-field of the L/ILW repository is characterised by strongly coupled hydro-chemical processes. From a post-closure safety point of view, ensuring a high-pH environment and favourable gas-related properties is key. A broad spectrum of experiments, from the molecular to the URL scale, along with their respective modelling, generally leads to converging outcomes in terms of safety-relevant impacts. While certain interactions can now be shown to be self-limiting (e.g. cement-clay interactions), some significant uncertainties remain for other issues, which will be difficult to reduce (e.g. related to the heterogeneity of the waste). These will be integrated into the safety analysis by defining bounding assumptions.
• Microbial investigations suggest that selected backfill materials (and possibly the EDZ) are likely to host microbial activity which, if it can be shown to have beneficial impacts with respect to gas mitigation, can potentially be optimised.

Knowledge of radionuclide transport in clay- and cement-based backfills has reached a high level of maturity, and the synthesis will be integrated into the sorption and diffusion databases for the safety case for the general licence applications. Monitoring – Monitoring studies conducted since 2016 have focused on concepts for underground and surface-based monitoring, strategic aspects of monitoring parameter selection and the link to safety analysis, as well as practical work on the development and testing of monitoring techniques and sensors. With the current status of the RD&D programme described in this report, Nagra is well placed to submit the documents for the general licence applications as planned in 2024 and to move forward with its repository programme.